The present invention pertains to an improved method for plugging or sealing selected zones in a subterranean formation. More particularly, this invention relates to an improved method for plugging such subterranean zones via the sequential injection therein of an aqueous solution of a water soluble silicate material, an aqueous spacer material and aqueous hydraulic cement slurry.
In the drilling, completion and servicing of oil and gas wells, it is known that selected zones of a subterranean formation can be sealed or plugged off by injecting therein an aqueous sodium silicate solution. In connection with such a treatment, gellation of said sodium silicate solution can be brought about by causing said solution to contact a suitable brine in the desired subterranean location or by the addition to said solution, prior to injection thereof into the formation, of a reagent which induces a controlled, gradual gellation of said sodium silicate solution.
It is also known that the aforementioned sodium silicate gel formation can be followed by the injection of a hydraulic cement slurry. However, since hydraulic cement slurries rapidly gel and set up upon contact with such sodium silicate material, it is common practice to employ a water spacer to separate the cement slurry and the silicate solution from each other during the injection thereof into the subterranean formation of interest.
While the use of a water spacer during injection to separate the cement slurry from the silicate solution can be reasonably effective in preventing premature admixture thereof, it is nonetheless still possible for said materials to prematurely come into contact with each other and to set up at an undesired location during injection such as, for example, in the wellbore itself or at an unintended position within the subterranean formation. Accordingly, it would be highly desirable to provide a means of controlling (e.g., delaying) the gellation rate exhibited by a mixture of such materials and to thereby ensure or obtain continued mobility or pumpability of such a mixture for a limited period of time after the individual silicate solution and cement slurry ingredients thereof come into direct contact with each other.